Method of plating copper particles with silver



Nov. 20, 1956 J. J. COLEMAN ETAL 2,771,380

' METHOD OF PLATING COPPER PARTICLES WITH SILVER Filed Aug. 2, 1954WATER- SOLUBLE BODIES COPPER PARTICLES e.g. GLAUBER'S SALT MIXED DRYAQUEOUS IMMERSION SILVER PLATING BATH SILVER PLATED COPPER PARTICLES INV EN TORS.

METHOD OF PLATING COPPER PARTICLES WITH SILVER Joseph J. Coleman andEmil M. Dittman, Freeport, Ill., assignors to Burgess Battery Company,Freeport, III., a corporation of Delaware Application August 2, 1954,Serial No. 447,409

8 Claims. (Cl. 117-227) This invention relates to a method for platingcopper particles with silver, and particularly to an immersion type ofmethod, as distinguished from an electrolytic method, of plating copperparticles with silver.

The immersion plating of copper with silver is accomplished by immersingthe articles to be plated in an aqueous cyanide silver plating bath, andin the plating of copper particles there is a tendency for the particlesto agglomerate or adhere together in the form of small masses orpellets. This has two undesirable effects, the first being that theagglomeration itself is objectionable, and the second being that thecopper particles are incompletely coated or plated with silver.

It is an object of the invention to provide a method for the immersionplating of copper particles with silver in which the objectionableagglomeration is avoided and complete and continuous coatings or platesof silver are obtained upon the individual copper particles.

The process of the invention is indicated by the flow sheet shown in thedrawing.

The method of the invention is applicable to copper particles of anyform including flakes. Such particles are susceptible to agglomerationand the method of the invention is effective in avoiding agglomerationof copper par ticles of any form including flakes during plating thereofwith silver. The method has been used with success with particles whichrange in size from about one millimeter to a few microns in theirmaximum dimension. silver coated copper particles of this character findusefulness as an ingredient in electrically conductive compositions, asfor example, in admixture with a binder or vehicle to form a conductivecomposition or paint.

In accordance with the invention, the copper particles are immersed inthe aqueous silver plating bath in the presence of a medium or agentwhich maintains the copper particles in separated or dispersedcondition. In one procedure, the separating agent is in the form ofdiscrete water-soluble bodies such as crystalline hydrated sodiumsulfate (Glaubers salt), potassium sulfate, sodium nitrate or the like.In a preferred procedure, the copper particles are first dry mixed withthe discrete bodies and the mixture is immersed in an aqueous cyanidesilver plating bath. The water-soluble bodies are larger than the copperparticles and the volume of said bodies is substantially greater thanthat of the copper particles, whereby the copper particles becomeattached to the surfaces of said bodies in separated or dispersedcondition and not in agglomerated condition. A suitable size for thewatersoluble bodies is from 0.25 inch to .0014 inch. The water-solublebodies may be in the form of preformed pellets, or crystals produced bycrystallization processes, or granules formed by the fracture of largerbodies.

As an example of the method, 30 parts by weight of flake copper is drymixed with 400 parts of bodies of hydrated sodium sulfate, Na2SO4-10H2O,in a suitable blender or mixer until the copper particles becomeattached in the form of a coating of dispersed particles to Small icethe surfaces of the sodium sulfate bodies. The mixture of flake copperand hydrated sodium sulfate bodies is then immersed in 800 parts of acyanide silver plating solution, constant agitation being maintainedduring the immersion. The plating bath may be of conventionalcomposition as follows, the proportions being in parts by weight:

Silver cyanide Potassium cyanide 125 Potassium carbonate 25 Water 1825Agitation is continued until the sodium sulfate becomes dissolved in theplating solution, when it is discontinued and the solution is allowed tostand until the particles have settled to the bottom of the container.During the agitation and solution of the sodium sulfate, a plating ofthe copper particles with silver takes place by a chemical deposition ofsilver from the silver cyanide in the plating bath. The silver-coatedparticles are separated from the plating bath in any suitable manner asby decantation and are then thoroughly washed with water and dried.

It has been found that in the process which has been described thesilver is deposited in the form of a complete and continuous coating orplate upon the surfaces of the individual copper particles. Theindividual particles are visible to the eye, thereby showing theirnon-agglomeration. The bright silver color of the particles is alsovisible to the eye, thereby showing the complete coating of theindividual particles with silver. Upon analysis the particles are foundto contain 12 percent by weight of silver.

To further demonstrate that complete silver coatings are produced uponthe individual copper particles by the process of the invention, aconductive composition was prepared containing the particles resultingfrom the method which is described in the foregoing example. Theparticles were thoroughly mixed with molten microcrystalline wax, theproportions being 14 parts by weight of microcrystalline wax and and 18parts of particles. The composition was then cooled to room temperaturewith solidification-of the microcrystalline wax, and was extruded underpressure into the form of a cylindrical section 4; inch in diameter and6 inches long. The resistance of the section was measured and found tobe less than one ohm.

For comparison purposes, similar copper particles were subjected to aplating method exactly the same as described in the foregoing exampleexcept that the hydrated sodium sulfate bodies were dispensed with. Theresulting plated copper was in the form of silver-coated masses orpellets which were round or nearly round in shape. The pellets werebroken open and the interiors were found to be composed of uncoatedcopper flakes which upon standing in 'the atmosphere turned dark incolor. The silver-coated pellets were subjected to a mild crushingoperation to separate the agglomerations into individual particles. Whenmixed with microerystalline wax in the manner described in theforegoing, the resulting composition exhibits an extremely highresistance such that for practical purposes it is a non-conductor.

The superior conductivity of the composition containing the particlesplated in accordance with the present invention is due to the fact thatthe copper particles are completely coated by silver whereas the otherparticles are not. Copper is subject to the acquisition in theatmosphere of a surface factor which greatly increases the contactresistance, whereas silver is substantially unaffected in this way.

Invention is claimed as follows:

1. The method of plating copper particles with silver which comprisesdry mixing said copper particles with discrete water-soluble bodieswhereby said copper particles become attached to the surfaces of saidwater-soluble bodies, said water-soluble bodies being of a sizesubstantially larger than that of said copper particles and the volumeof the water-soluble bodies being greater than that of the copperparticles and being sufficient that the copper particles are indispersed condition on the surfaces of the water-soluble bodies,immersing said mixture in an immersion type aqueous silver plating bath,causing said water-soluble bodies to be dissolved in said plating bath,and separating the resulting silver plated copper particles from saidplating bath.

2. The method of plating copper particles with silver which comprisesdry mixing said copper particles with discrete water-soluble bodieswhereby said copper particles become attached to the surfaces of saidwatersoluble bodies, said water-soluble bodies being of a sizesubstantially larger than that of said copper particles and the volumeof the water-soluble bOdies being greater than that of the copperparticles and being suflicient that the copper particles are indispersed condition on the surfaces of the water-soluble bodies,immersing said mixture in an immersion type aqueous silver plating bath,and causing said water-soluble bodies to be dissolved in said platingbath.

3. The method of plating copper particles with silver which comprisesdry mixing said copper particles with discrete water-soluble bodieswhereby said copper particles become attached to the surfaces of saidwater-soluble bodies, immersing said mixture in an immersion typeaqueous silver plating bath, and causing said water-soluble bodies to bedissolved in said plating bath.

4. The method of plating copper particles with silver which comprisesimmersing said copper particles in an immersion type aqueous silverplating bath while attached to the surfaces of undissolved water-solublebodies of a size greater than that of said copper particles.

5. The method of plating copper particles with silver which comprisesdry mixing said copper particles with discrete bodies of crystallinehydrated sodium sulfate whereby said copper particles become attached tothe surfaces of said sodium sulfate bodies, said sodium sulfate bodiesbeing of a size substantially larger than that of said copper particlesand the volume of sodium sulfate bodies being greater than that of thecopper particles and being sufficient that the copper particles are indispersed condition upon the surfaces of the sodium sulfate bodies,immersing said mixture in an immersion type aqueous silver plating bath,causing said sodium sulfate to be dissolved in said plating bath, andseparating the resulting silver plated copper particles from saidplating bath.

6. The method of plating copper particles with silver which comprisesdry mixing said copper particles with bodies of crystalline hydratedsodium sulfate whereby said copper particles become attached to thesurfaces of said sodium sulfate bodies, said sodium sulfate bodies beingof a size substantially larger than that of said copper particles andthe volume of sodium sulfate bodies being greater than that of thecopper particles and being sufficient that the copper particles are indispersed condition and are not agglomerated upon the surfaces of thesodium sulfate bodies, immersing said mixture in an immersion typeaqueous silver plating bath and causing said sodium sulfate to bedissolved in said plating path.

7. The method of plating copper particles with silver which comprisesdry mixing said copper particles with bodies of crystalline hydratedsodium sulfate whereby said copper particles become attached to thesurfaces of said sodium sulfate bodies, immersing said mixture in animmersion type aqueous silver plating bath, and causing said sodiumsulfate to be dissolved in said plating bath.

8. The method as claimed in claim 4 in which the copper particles are inthe form of flakes.

References Cited in the file of this patent UNITED STATES PATENTS1,208,507 Dalby Dec. 12, 1916 2,110,792 Egeberg et al. Mar. 8, 19382,113,517 Powell et al. Apr. 5, 1938 2,391,289 Beaver Dec. 18, 19452,472,393 Avallone et al. June 7, 1949 2,613,179 Wolfson et al. Oct. 7,1952 FOREIGN PATENTS 396,250 Great Britain Aug. 3, 1933 OTHER REFERENCESSanigar: The Electrochemical Society, Preprint 5933, April 27, 1931, pp.363 to 367.

1. THE METHOD OF PLATING COPPER PARTICLES WITH SILVER WHICH COMPRISESDRY MIXING SAID COPPER PARTICLES WITH DISCRETE WATER-SOLUBLE BODIESWHEREBY SAID COPPER PARTICLES BECOME ATTACHED TO THE SURFACES OF SAIDWATER-SOLUBLE BODIES, SAID WATER-SOLUBLE BODIES OF A SIZE SUBSTANTIALLYLARGER THAN THAT OF SAID COPPER PARTICLES AND THE VOLUME OF THEWATER-SOLUBLE BODIES BEING GREATER THAN THAT OF THE COPPER PARTICLES ANDBEING SUFFICIENT THAT THE COPPER PARTICLES ARE IN DISPERSED CONDITION ONTHE SURFACES OF THE WATER-SOLUBLE BODIES, IMMERSING SAID MIXTURE IN ANIMMERSION TYPE AQUEOUS SILVER PLATING BATH, CAUSING SAID WATER-SOLUBLEBODIES TO BE DISSOLVED IN SAID PLATING BATH, AND SEPARATING THERESULTING SILVER PLATED COPPER PARTICLES FROM SAID PLATING BATH.